This second round of investigation stemmed from concerns about observing more than just the inner, high surface brightness regions of galaxies.
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NGC 6946
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NGC 6946 serves as our test case.
The series of 9"-spaced plus symbols indicates one potential radial cut
along the major axis of NGC 6946. The symbols in this figure span
a radial distance of 225", or approximately the length of 1.5 long-low
subunits of IRS. The outermost regions of the cut show a HI column
density of ~1e21 H-atoms/cm^2 (Tacconi & Young 1986). This is
about a factor of four larger than the HI column density at high Galactic
latitudes (Kularni & Heiles 1988).
These regions have a 15 micron surface brightness of ~1 MJy/ster. Interestingly, If we assume a radiation field comparable to that of the local ISRF and a Milky Way metallicity, the inferred atomic gas density that we are sampling is also ~1e21 H-atoms/cm^2.
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Results
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Spectra for the cut along NGC 6946
are displayed in MJy/ster. The eight brightest spectra are from the
inner 75"; the 17 weaker spectra correspond to radial distances between
75" and 225". Notice how the spectral shapes differ for the different
regions. These trends were estimated using the ISO 7/15 micron ratio
and a model for normal galaxy SEDs. As a reminder, the entire span
from x=-225" to x=+225" could be covered by the length of three long-low
subunits placed end-to-end.
Signal-to-noise estimates show that we can recover adequate data from the central regions using a 14 second elementary (scan) integration. Longer integrations would be required to achieve reasonable S/N for lower surface brightness regions. S/N estimates are provided for 30, 60, 120, and 240 second elementary (scan) integrations.
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Discussion
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Note that the lowest S/N coverage occurs for the 5.3-7.5 micron short-low
subunit. Moreover, recall that the bulk of the `action' occurs at
longer wavelengths: (i) the steep mid-IR slopes that are typical
of HII region cores are most clearly evident beyond ~ 9 microns (e.g Cesarsky
et al. 1996); (ii) the clusters of PAH features near 8 microns and 12 microns
are also not covered by the short-low subunit.
It may behoove us to focus on the 7.5 to 40 micron wavelength span, and maybe even just 14.2 to 40 microns for observations of normal galaxies. Since the 5.3 to 7.5 micron wavelength regime is relatively `unexciting' and will be covered by the IRAC 5.8 micron array observations, we can argue that we need not spend valuable time observing with this subunit. This will cut down IRS integrations times by nearly a factor of two (see table).
The first observing figure outlines one possible observing strategy. Each pair of rectangles represents the area covered by one observing component. The top four pairs outline the 151.3"x54.6" rectangulars areas we may plan on observing with the long-low subunits, whereas the bottom four pairs of 54.6"x54.6" squares show possible short-low target fields. The second observing figure summarizes the wavelength coverage for the observed positions.
The integrations indicated in the first observing figure assume we would use Exptime=30 seconds for long-low, Exptime=14 seconds for `nuclear' short-low, and Exptime=60 seconds for outer-disk short-low observations. This should ensure good S/N, for 7.5 to 40 microns, throughout the central 450"x54.6" area.
Finally, I have highlighted one possible short-low observing strategy. Because short-low integrations are relatively expensive, we could decide in advance which regions are attractive (e.g. nucleus, HII regions, or high surface brightness at a particular wavelength), and then hop-skip our way across the region observed with long-low subunits.
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Summary
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The following table shows the integration times necessary to map the
450"x54.6" major axis region from 7.5 to 40 microns (partial maps of satellite
fields will come for free).
Exptime Scan rate Scan Length Scan Integration
(sec) (arcsec/sec) (arcsec)
(minutes)
Short-low 6
0.13846 54.6
6.6
14 0.08571
10.6
60 0.02432
37.4
240 0.00608
149.7
Long -low 6
0.37308 54.6
2.4
14 0.23095
3.9
30 0.13108
6.9
120 0.03277
27.8
The total integration time for this possible strategy would be 2x10.6 + 2x 37.4 + 4x6.9 = 123.6 minutes. We need not restrict ourselves to 54.6" scans, the width of the short-low subunits. We could elect to do short 30" scans, for example. Moreover, we may elect to do only 2 or 3 short-low scans.
Danny Dale
19 May 2000
